A typical loudspeaker enclosure is a source for a sound field. For example, a typical loudspeaker enclosure may be used to generate a sound field for “live” sound reinforcement, for home entertainment, for car audio, for a discothèque, or the like. Generally, three-dimensional radiation patterns of sound fields generated by a typical loudspeaker vary with frequency. Such a sound field also may not be focused, and spectral content of the sound field may vary with direction. In applications where a sound filed is generated in an enclosed or a partially enclosed space, an unfocused sound field may cause constructive and destructive wave interference patterns, which may further distort the sound field at different locations.
A theoretically ideal loudspeaker, on the other hand, produces a sound field with a spectral content that does not vary with direction, and which has three-dimensional radiation patterns that are constant over a wide frequency range. For certain applications, such as use in an enclosed or partially enclosed space, it may be desirable to have a loudspeaker that has constant directivity in addition to radiation patterns that are constant over a wide frequency range. A loudspeaker with radiation patterns that do not differ significantly with respect to frequency is referred to herein as a constant-directivity or a constant-beamwidth loudspeaker.
Various methods have been used in the sound industry to attempt to construct a constant-beamwidth loudspeaker that overcomes the above mentioned problems. The use of horns, arrays and higher order sources have all been implemented. In sonar applications, constant-beamwidth transducers using spherical caps have been described in the literature. So far, none of these approaches have overcome the problems described above that are associated with typical loudspeakers. It would be desirable to provide a constant-beamwidth loudspeaker that produces a sound field with spectral content that does not vary significantly with direction and that has three-dimensional radiation patterns that are relatively consistent over a wide frequency range.